Licking and chinning can be part of marking territory.
I wouldn't be concerned unless you use a cleansing product on the metal in which case I would make sure it's animal safe.
Has your bunny got a friend or do your buns live seperately? Normally licking is seen between rabbits and often we see it when one of our fosters is 'ready for a friend'. It can be a signal that the rabbit needs companionship.
If they are already together then I'd say it's about heirachy (unless there is something tasty on the stool)